1. Filed of the Invention
This invention relates to a magnetic recording medium and a process for producing the same. More particularly, it relates to a multi-layered magnetic recording medium and a process for producing the same, in which the stiffness is controlled, particularly the stiffness in the width direction is improved, to improve various required characteristics, such as improved output by a head touch, tape exchangeability, adequate bending stiffness for prevention of edge cracks, and reduced fluctuations in slit width direction.
2. Description of the Related Art
Stiffness of a magnetic recording medium has conventionally been controlled by using a base film (substrate) having different Young's modulus in its longitudinal direction and width direction. Where a substrate has on its reverse side a backcoating layer, the stiffness is also controllable by changing the thickness of the backcoating layer.
However, a change of the stiffness of a base film or of the thickness of a backcoating layer is liable to present difference problems. For example, to use a base film whose ratio of stiffness in the longitudinal direction to that in the width direction has been changed decreases the yield and increases the cost. Thickening of a backcoating layer tends to result in insufficient drying because of the low solids content of the coating composition.
When a coating composition containing acicular particles is applied by a usual coating technique, such as doctor blade coating, gravure coating, reverse roll coating, die coating, etc., a shear force is imposed on the coating composition so that the particles are oriented in the longitudinal direction. This being the case, the resulting magnetic recording medium has insufficient stiffness in the width direction for improving the above-mentioned characteristics.
T. Masuko, J. Sugawara, and K. Iwakura report in Repts, Progr. Phys. Jpn., 38, in press (1986) that acicular magnetic particles are oriented in the shear direction with a shear force stronger than the cohesive force applied thereto. Nonmagnetic particles in a coating composition are similarly oriented in the longitudinal direction by imposing a shear force on the coating composition in the longitudinal direction on application as disclosed in Japanese Patent Laid-Open No. 274855/194. However, it is desirable for a magnetic disc or a magnetic sheet to be isotropic in stiffness. Although magnetic particles could be made isotropic by random orientation, it is difficult to isotropically orient nonmagnetic particles by magnetic field orientation.
It is important for a magnetic tape to have well-balanced stiffness in its longitudinal direction and width direction. In using acicular ferromagnetic powder, however, the stiffness becomes higher in the longitudinal direction because the particles should be oriented in the longitudinal direction in order to obtain high outputs in electromagnetic conversion characteristics at the sacrifice of stiffness in the width direction.
Japanese Patent Laid-Open Nos. 314551/95, 90648/96, and 126828/94 propose using, as a base film, a biaxially stretched polyester film which has an improved Young's modulus in the width direction thereby to improve the stiffness in the width direction. Japanese Patent Laid-Open No. 78630/82 teaches improvement comprising using acicular ferromagnetic particles in a lower layer and applying a magnetic field in the width direction to orient the particles in that direction. Japanese Patent Laid-Open Nos. 282836/94 and 243460/94 disclose similar techniques, in which acicular hexagonal magnetic ferrite particles are oriented in a lower layer by magnetic field treatment so as to have their major axis in parallel to the width direction. Further, a method for controlling the stiffness balance in the longitudinal and width directions by using polyhedral inorganic particles having a spherical to cubic shape is disclosed in Japanese Patent Laid-Open No. 335310/96. These conventional techniques are disadvantageous in that high cost is incurred or the effects produced are not sufficient because of the spherical shape of the particles. Unlike these techniques, the idea of the present invention consists in controlling the degree of orientation of acicular nonmagnetic particles of an intermediate layer by the shape of the particles, the direction of shear imposed to the coating composition immediately before application, and dispersion of the coating composition.
Japanese Patent Laid-Open No. 326043/95 discloses a magnetic recording medium containing acicular .alpha.-Fe.sub.2 O.sub.3 as the main particles of a lower layer having added thereto a slight amount of magnetic powders so as to have the orientation in the longitudinal direction, whereby the orientation of filler in the lower layer is strengthen and the smoothness of the boundary surface between upper and lower layers and the top surface of a magnetic layer is improved. In this publication, the longitudinal direction is referred to in contrast to the perpendicular direction. On the other hand, the present invention is to improve the film stiffness by rendering the orientation in the longitudinal and width directions in the surface thereof closer to random orientation. In the present direction the longitudinal direction in the surface direction is referred to in contrast to the width direction.